The invention concerns a method for producing a multi-step structure in a substrate.
In many technical fields it is necessary to produce a step structure in a substrate. Single-step structures are generally produced by removing substrate with the aid of, for example, mechanical, physical or chemical processes. For larger numbers and higher accuracy requirements, photoprocesses are frequently employed for defining the location of the step on the substrate. In such processes, a photoactive substance is deposited on the structure, exposed through a mask, and developed, so that areas for the removal of substrate are passivated or substrate is removed through a mask overlying the substrate during removal.
For producing a multi-step structure, a sequential approach is normally adopted. Initially, a single-step structure is formed, in which further steps are produced. This necessitates however that the first step in the substrate has a uniform depth so that irregularities existing therein are not repeated in the next steps. Apart from this, it is difficult with this approach to position the second photomask and inaccuracies may occur when the second step is formed in the substrate. Thus, this approach is only suitable for steps of a particular size for which the accuracy requirements are not too exacting.
Such step structures are frequently used for computer components, such as the wiring planes of semiconductor chips or the thin-film wiring on multilayer ceramic substrates.
As components of this type are being increasingly miniaturized and have to meet ever higher accuracy requirements, the previously described approach has reached its limits so that a new concept had to be found for producing such substrates.
From IBM Technical Disclosure Bulletin, Vol. 33, No. 2, July, 1990, p. 447 "Dual-Image Resist for Single-Exposure Self-Aligned Processing" a photoresist system is known which consists of a wet developable first and a dry developable second photoresist which are radiated at different wavelengths. Such a system may be used to fabricate step-shaped structures from a substrate by exposure to a first wavelength and development, followed by a second exposure, development and processing after the first opening has been formed. This approach is limited however since photoresists responsive to different wavelengths are expensive and the number of steps thus produced cannot be increased at random, not to mention the fact that the precision obtainable depends on how sensitive the second photoresist is to the first wavelength.